1. Field of the Invention
The present invention relates to an image forming apparatus, such, as a printer or a copier, and to an image forming method that uses an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
In recent years, image forming apparatuses that use an electrophotographic method have been made with increased speed and performance, and have also progressed in color image forming technology. Printers and copiers that use a variety of methods to form color images have been brought onto the market. For example, a so-called “inline-type” image forming apparatus is known, in which image forming means for a plurality of colors are arranged in a line and toner images that have been formed by each of these image forming means are transferred in order on top of one another onto a sheet (the transfer medium). Image forming apparatuses of the inline type can form color images at high speeds, and so are expected to become the principal type of color printer, in the future. Such inline-type image forming apparatuses are constructed so as to hold and convey a sheet using a belt-shaped conveying means (a sheet conveyor belt) and to transfer toner images of colors on top of one another onto the sheet in order, thereby producing a color image.
In addition, in the inline-type image forming apparatus, the sheet conveyor belt extends in the direction in which image forming stations are arranged and is disposed in contact with each of the image forming stations. To improve the transferring of toner images onto a sheet that is conveyed by the sheet conveyor belt, it is preferable for the sheet to be stably held on the sheet conveyor belt by attraction. For example, by applying an attraction bias (a voltage or current) to the surface of the sheet conveyor belt, a sheet can be stably held on the sheet conveyor belt by attraction.
When forming toner images on both surfaces of a sheet using the inline-type image forming apparatus described above, after the trailing end of a sheet that has had an image formed on a first surface (the front) thereof has passed a fixer that heat-fixes the toner images on the sheet, the conveying direction of the sheet is reversed so that the second surface (the reverse) of the sheet becomes the image forming surface, the sheet is guided to a re-conveying path once again, and the sheet is fed towards the plurality of image forming means for a second time.
When images are formed on both surfaces of a sheet, however, image formation is performed on the second surface of the sheet after the sheet has been heated and subjected to pressure by the fixer, so that there are cases where a peripheral part of the sheet becomes wrinkled. If a sheet in this state is conveyed by the sheet conveyor belt to which an attraction bias has been applied, the wrinkled part of the sheet is strongly held on the sheet conveyor belt by attraction. As a result, as shown in FIGS. 5A and 5B, the wrinkled part)(Sb) of the sheet (S) causes creases to be formed that are directed towards the trailing end of the sheet. Toner images are not transferred to the concave parts of the creases (i.e., the parts where there is no contact between the sheet and the photosensitive drums), so that there are cases where certain areas are left unprinted. It should be noted that concave and convex parts of the creases are depicted in an exaggerated fashion in FIGS. 5A and 5B for the sake of explanation.
Also, when image formation has ended for the first surface (the front), wrinkling occurs in the sheet that has passed the fixer at both sides in the direction that is perpendicular to the conveying direction. The lengths in the conveying direction of the both sides at which such wrinkling occurs will be longer than the length of the central part of the sheet. This means that when image formation is performed on the second surface (the reverse) of the sheet, the wrinkled parts at both sides of the sheet cause creases to be formed concentrated at the trailing end of the sheet. This can result in unprinted areas being left on the sheet at the transfer sections.